The present invention relates to commercial processes for the production of antiviral 1,3-oxathiolane nucleoside analogues including, but not limited to, cis(xe2x88x92)-4-amino-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-2(1H)-pyrimidinone (emtricitabine, (xe2x88x92)-FTC, 1) and cis(xe2x88x92)-4-amino-1-[2-(hydroxymethyl)-1,3-oxathiolan-5-yl]-2(1H)-pyrimidinone (lamivudine, 3TC, 2). In one embodiment, the present invention relates to novel crystalline intermediates useful in the preparation of antiviral 1,3-oxathiolane nucleoside analogues in enantiomerically enriched form, and to commercial processes for their preparation. 
Nucleoside analogues are well known therapeutic agents, often displaying antiviral activity against retroviruses such as human immunodeficiency virus (HIV), hepatitis B virus (HBV), and human T-lymphotropic virus (HTLV). Generally speaking, nucleoside analogues can exist in two distinct stereoisomeric forms, known as cis and trans diastereomers. However, it is usually only the cis diastereomers of nucleoside inhibitors which display significant biological activity. Each cis and trans diastereomer is further composed of two pairs of stereoisomers, known as enantiomers. For many nucleoside analogues, including (xe2x88x92)-FTC and 3TC, the antiviral activity is significantly more pronounced in one of two possible enantiomeric forms of the cis diastereomer. In the case of (xe2x88x92)-FTC and 3TC, the levorotatory, or (xe2x88x92), enantiomer is the major contributor to the desired antiviral activity, as is disclosed in the following: U.S. Pat. Nos. 5,486,520; 5,539,116; J. Org. Chem. 1992, 57, 2217-2219; J. Med. Chem. 1993, 36, 181-195. Consequently, commercial manufacturing methods which are capable of producing nucleoside analogues in enantiomerically enriched form are of primary importance for the continued chemical and pharmaceutical development of such 1,3-oxathiolane nucleoside analogues.
The prior art discloses several methods for the preparation of 1,3-oxathiolane nucleoside analogues by processes which do not address the issue of enantioselectivity and, consequently, provide 1,3-oxathiolane nucleoside analogues as mixtures of enantiomers. Examples of such methods for obtaining 1,3-oxathiolane nucleoside analogues as diastereomeric and/or enantiomeric mixtures may be found in U.S. Pat. No. 5,204,466 xe2x80x9cMethod and Compositions for the Synthesis of BCH-189 and Related Compoundsxe2x80x9d, U.S. Pat. No. 5,210,085 xe2x80x9cMethod for the Synthesis, Compositions, and Use of 2xe2x80x2-Deoxy-5-fluoro-3xe2x80x2-thiacytidine and Related Compoundsxe2x80x9d, U.S. Pat. No. 5,466,806 xe2x80x9cProcess for Preparing Substituted 1,3-Oxathiolanes with Antiviral Propertiesxe2x80x9d and U.S. Pat. No. 5,763,606 xe2x80x9cStereoselective Synthesis of Nucleoside Analogues Using A Bicyclic Intermediatexe2x80x9d. Such processes have obvious limitations for commercial production purposes since the degree of enantioselectivity imparted into the finished active compound is marginal or absent.
On the other hand, U.S. Pat. No. 5,728,575 (xe2x80x9cMethod of Resolution of 1,3-Oxathiolane Nucleoside Enantiomersxe2x80x9d) describes the preparation of nucleoside analogues, including (xe2x88x92)-FTC, in enantiomerically enriched form using a process involving enzyme mediated hydrolysis of a racemic mixture of nucleoside analogue esters. Enzymes employed are selected from the group consisting of pig liver esterase (PLE), pig pancreatic lipase (PPL), and substillisin. Esters are selected from the group consisting of acetate, propionate, butyrate, and pentanoate. The enzyme preferentially catalyzes the hydrolysis of an ester group in one of the two enantiomers, allowing a subsequent separation of unhydrolyzed nucleoside analogue ester and hydrolyzed nucleoside analogue. The process can be depicted as follows: 
While the degree of recognition displayed by the enzyme for hydrolysis of one enantiomer over the other is very high, the resolution of enantiomers occurs at a very late stage in the overall process to the nucleoside analogue. This is an undesirable situation from a commercial production and economic standpoint because the resolution of enantiomers of the 1,3-oxathiolane nucleoside analogue occurs in the penultimate step of the process therefore results in, at minimum, fifty percent loss of the batch material. Secondly, enzymatic hydrolysis occurs on the undesired enantiomer preferentially, which necessitates a subsequent hydrolysis step on the separated desired enantiomer to completely form the desired nucleoside analogue. The consequences of this additional step are increased manufacturing overhead costs and loss of material through non-stoichiometric conversion. Furthermore, the possibility of recovering the undesired enantiomer and recycling it into the process to form the desired enantiomer becomes highly improbable to complete in an economical manner due to the inherent challenge of simultaneously epimerizing the two stereocentres from one cis conformation to the second (desired) cis conformation. Finally, the introduction of impurities in the finished product, originating from the enzyme preparation itself, is a major concern since the enzymatic resolution occurs in the penultimate step of the process.
Published PCT application WO 00/09494 discloses processes for the preparation of 1,3-oxathiolane nucleoside analogues which, when used in conjunction with the prior art, may be used to produce enantiomerically enriched 1,3-oxathiolane nucleoside analogues, including (xe2x88x92)-FTC.
U.S. Pat. No. 5,663,320 xe2x80x9cProcesses for the Diastereoselective Separation of Nucleoside Analogue Synthetic Intermediatesxe2x80x9d, U.S. Pat. No 5,684,164 xe2x80x9cProcesses for Preparing Substituted 1,3-Oxathiolanes With Antiviral Propertiesxe2x80x9d, U.S. Pat. No. 5,693,787 xe2x80x9cIntermediates in the Synthesis of 1,3-Oxathiolanyl Cytosine Nucleoside Analoguesxe2x80x9d, U.S. Pat. No. 5,696,254 xe2x80x9cProcesses for the Diastereoselective Synthesis of Nucleoside Analoguesxe2x80x9d U.S. Pat. No. 5,744,596 xe2x80x9cNucleoside Analogues and Synthetic Intermediatesxe2x80x9d, and U.S. Pat. No. 5,756,706 xe2x80x9cProcesses for the Diastereoselective Synthesis of Nucleoside Analoguesxe2x80x9d, describe processes for the enantioselective preparation of nucleoside analogues, including 1,3-oxathiolane nucleoside analogues. For example, one such process involves the use of enantiomerically enriched intermediates of the form 
where R is a chiral auxiliary, and L is a leaving group. Preferred chiral auxiliaries in this case are selected from (+) and (xe2x88x92)-menthol and (+) and (xe2x88x92)-norephedrine. Prior to the conversion of such intermediates into 1,3-oxathiolane nucleoside analogues, reduction of the substituted carbonyl group to a hydroxymethyl group is required. This may lead to undesired racemization of the 1,3-oxathiolane nucleoside analogue during chemical reduction thus resulting in economic and material loss, and necessitating further processing to arrive at material of pharmaceutically acceptable purity.
An improved method for the obtention of 1,3-oxathiolane nucleoside analogues is therefore required as there continues to exist an ongoing requirement to produce the therapeutically important antiviral agents, such as (xe2x88x92)-FTC and 3TC, using safe, efficient, and economical commercial processes avoiding the drawbacks of the prior art discussed above.
It is therefore an object of the invention to provide an improved process for obtaining 1,3-oxathiolane nucleoside analogues and intermediates useful in the obtention thereof which overcomes the disadvantages of the prior art.
Furthermore, it is an object of the invention to provide novel intermediates useful in the process for obtaining the 1,3-oxathiolane nucleoside analogues and intermediates useful in the obtention thereof. It is still a further object of the invention to provide novel intermediates useful in the manufacture of the 1,3-oxathiolane nucleoside analogues. It is still a further object of the invention to provide a process for the manufacture of the novel intermediates in enantiomerically enriched form.
Further and other objects of the invention will be apparent to those skilled in the art from the following summary of invention and the detailed description of embodiments thereof.
It has now been found that compounds of the general formula A 
wherein R* is a chiral auxiliary, and preferably in one instance, the configuration at the asymmetric carbon atom between oxygen and sulfur is (R), (S), or combinations of (R) and (S), and preferably within the scope of compounds within the formula A, compounds of the general formula B 
wherein R3 is either a chiral or an achiral auxiliary, and preferably in one instance the configuration at the asymmetric carbon atom between oxygen and sulfur is (R), (S), or combinations of (R) and (S), are useful in the production of 1,3-oxathiolane nucleoside analogues, as described below. The substituents R* and R3 are such that they permit the compounds (A) or (B) to react in the processes to make 1,3-oxathiolane nucleoside analogues or any intermediate useful in the manufacture of 1,3-oxathiolane nucleoside analogues and do not interfere in the manufacture thereof.
Many techniques such as chromatographic separation or distillative separation are known to be useful in the non-chemical based resolution of stereoisomers. By industrial practice, the obtention of enantiomerically enriched pharmaceutical intermediates and active ingredients continues to rely heavily upon selective crystallization of diastereomeric compounds because of economic advantages and the use of standardized equipment (see J. Jacques et al., Enantiomers, Racemates, and Resolutions, John Wiley and Sons, New York, 1981). Crystallization based processes are especially desirable when the possibility of recovering and recycling of undesired stereoisomers exists. The discovery of such novel intermediates A and B, crystalline in form, and their purification via recrystallization techniques, allows the removal of their undesired stereoisomers and of other process derived impurities which might be otherwise difficult to remove at later stages of commercial manufacturing.
Therefore, according to one aspect of the invention, a process for the industrial synthesis of intermediates useful in the preparation of 1,3-oxathiolane nucleoside analogues, in enantiomerically enriched form, is disclosed. The discovery of such novel, enantiomerically enriched intermediates permits their coupling to a variety of base partners to provide 1,3-oxathiolane nucleoside analogues, including, but not limited to, (xe2x88x92)-FTC and 3TC, in enantiomerically enriched form.
Therefore, according to one aspect of the invention there is provided a compound of the formula A, 
wherein R* is a chiral auxiliary, preferably the configuration at the asymmetric carbon atom between oxygen and sulfur is (R), (S), or combinations of (R) and (S), wherein compound of formula (A) is useful in the preparation of enantiomerically enriched 1,3-oxathiolane nucleoside analogues.
According to another aspect of the invention, there is provided a compound of formula B, 
wherein R3 is a chiral or achiral auxiliary, preferably the configuration at the asymmetric carbon atom between oxygen and sulfur is (R), (S), or combinations of (R) and (S), wherein the compound of formula (B) is useful in the preparation of enantiomerically enriched 1,3-oxathiolane nucleoside analogues.
According to another aspect of the invention, there is provided a process for the manufacture of a compound of the formula A 
wherein R* is a chiral auxiliary, preferably the configuration at the asymmetric carbon atom between oxygen and sulfur is (R), (S), or combinations of (R) and (S), said process comprising the following: 
Preferably the compound of formula A is 
wherein R3 is selected from the group consisting of a chiral or achiral auxiliary, preferably the configuration at the asymmetric carbon atom between oxygen and sulfur is (R), (S), or combinations of (R) and (S). Preferably said compound is 
Preferably the chiral or achiral auxiliary permits the compound to react in the process to make a 1,3-oxathiolane nucleoside analogue or any intermediate useful in the manufacture of a 1,3-oxathiolane nucleoside analogue. Preferably the chiral auxiliary is a chiral auxiliary derived from a carboxylic acid or derivatives thereof. Preferably the chiral auxiliary is selected from the group consisting of mandelic acid, menthoxyacetic acid, amino acids, camphorsulphonic acid, 2-arylpropionic acids, enantiomers thereof, preferably enantiomerically enriched forms thereof and derivatives thereof.
According to yet another aspect of the invention there is provided use of a compound of the formula A 
wherein R* is a chiral auxiliary, preferably the configuration at the asymmetric carbon atom between oxygen and sulfur is (R), (S), or combinations of (R) and (S), in the preparation of enantiomerically enriched 1,3-oxathiolane nucleoside analogues or derivatives thereof or intermediates useful in the manufacture of 1,3-oxathiolane nucleoside analogues. Preferably the 1,3-oxathiolane nucleoside analogues or derivatives thereof or intermediates useful in the manufacture of 1,3-oxathiolane nucleoside analogues are enantiomerically enriched. Preferably the compound of formula A is 
wherein R3 is selected from the group consisting of a chiral or achiral auxiliary, preferably the configuration at the asymmetric carbon atom between oxygen and sulfur is (R), (S), or combinations of (R) and (S). More preferably the compound of formula A is 
According to yet another aspect of the invention, there is provided a process for the manufacture of compound of formula 
useful as an intermediate for conversion into an enantiomerically enriched 1,3-oxathiolane nucleoside analogue, comprising the reaction of mandelic acid with a compound of formula 4 
wherein X is a leaving group, preferably halogen, more preferably chlorine or bromine, to produce a compound of formula 5, 
and thereafter converting the compound of formula 5 into the intermediate of formula 9 preferably in a substantially high purity, crystalline form, preferably by the steps of halogenation, esterification, lactonization, and selective crystallization. Preferably the step of halogenation involves conversion of (2S)-2-(trimethylacetyloxy)phenylacetic acid into a (2S)-2-(trimethylacetyloxy) phenylacetyl halide; preferably the step of esterification involves reacting the (2S)-2-trimethylacetyloxy)phenylacetyl halide with a 2,2-dialkoxyethanol compound to produce a 2,2-dialkoxyethyl (2S)-2-(trimethylacetyloxy)phenylacetate; preferably the step of lactonization involves reacting the 2,2-dialkoxyethyl (2S)-2-(trimethyl-acetyloxy)phenylacetate intermediate with 2-mercaptoacetic acid to produce a mixture of (2R)-2-[(2xe2x80x2S)-2xe2x80x2-(trimethylacetyloxy)phenylacetyloxymethyl]-1,3-oxathiolan-5-one and (2S)-2-[(2xe2x80x2S)-2xe2x80x2-(trimethylacetyloxy)phenylacetyloxy methyl]-1,3-oxathiolan-5-one; and preferably the step of selective crystallization involves the separation of (2R)-2-[(2xe2x80x2S)-2xe2x80x2-(trimethylacetyloxy)phenylacetyloxymethyl]-1,3-oxathiolan-5-one from (2S)-2-[(2xe2x80x2S)-2xe2x80x2-(trimethylacetyloxy)phenylacetyloxymethyl]-1,3-oxathiolan-5-one mixture to produce enantiomerically and diastereomerically enriched (2R)-2-[(2xe2x80x2S)-2xe2x80x2-(trimethylacetyloxy)phenylacetyloxy methyl]-1,3-oxathiolan-5-one; preferably the trimethylacetyl halide is trimethylacetyl chloride; preferably still the (2S)-2-(trimethylacetyloxy)phenylacetyl halide is (2S)-2-(trimethylacetyloxy)phenylacetyl chloride.
According to yet another aspect of the invention, there is provided (2R) and (2S)-2-[(2xe2x80x2S)-2xe2x80x2-(Trimethylacetyloxy)phenylacetyloxymethyl]-1,3-oxathiolan-5-one.
Preferably the intermediate of formula 9 produced above is further converted to the desired 1,3-oxathiolane nucleoside analogue, preferably 3TC or (xe2x88x92)-FTC.
According to yet another aspect of the invention there is provided a process for converting an intermediate of formula (A) to a 1,3-oxathiolane nucleoside analogue preferably by the process which involves the steps of acylation, preferably reductive acylation, coupling with a base, and hydrolysis. Preferably the 1,3-oxathiolane nucleoside analogue is (xe2x88x92)-FTC, or 3TC.
According to yet another aspect of the invention there is provided a process for the recovery and recycling of (2S)-2-[(2xe2x80x2S)-2xe2x80x2-(trimethylacetyloxy) phenylacetyloxymethyl]-1,3-oxathiolan-5-one.
According to yet another aspect of the invention there is provided a process of preparing a compound of the formula (2R)-A comprising the following: 
According to yet another aspect of the invention there is provided use of a compound of the formula (2R)-A in the manufacture of a 1,3-oxathiolane nucleoside analogue or enantiomerically enriched intermediate thereof.
According to yet another aspect of the invention there is provided a process of preparing a 1,3-oxathiolane nucleoside analogue comprising the following: 
According to yet another aspect of the invention there is provided a process of preparing a 1,3-oxathiolane nucleoside analogue comprising the following: 